14.1 Work and Power
CHAPTER 14: WORK, POWER, AND
MACHINES
What is Work?
 Define Work:

The product of force and distance.
Work Requires Motion
 What condition must exist in order for a force to do
work on an object?

Some of the force must act in the same direction as the object
moves.
 If there is no movement, no work is done.
Work Depends on Direction
 The amount of work done on an object depends on
what two things?


Direction of the force
Direction of the movement
 Does all the force have to act in the direction of
movement to do work?

No
 Think of an original idea where this is true.
Calculating Work
 How do you calculate work?

Multiply the force acting in the direction of motion by the
distance the object moves

Work = Force x Distance

Tomorrow when I am gone, you will do math practice with this
formula 
Units of Work
 What are the two ways you can label work?
 Newton * Meter


force (N) x distance (m)
Joule (J)

1 N*m = 1 J
Using the Work Formula
 How much work does a 25 N force do to lift a potted
plant from the floor to a shelf 1.5 m high?

Work = Force x Distance

Work = 25 N x 1.5 m

Work = 37.5 N*m

Work = 37.5 J
What is Power?
 Define Power:

The rate of doing work.
 What are two ways to increase power?

Increase the amount of work done in a given time.

Do a given amount of work in less time.
 How does doing work at a faster rate affect the power
required?

More power is required.
 Think of an original example of two things that do
the same task, where one requires more power, but
less time and the other requires less power, but more
time.


Plowing a field with a horse & chisel
Plowing a field with a tractor
Calculating Power
 How do you calculate power?
 Divide the amount of work done by the time needed to do the
work.

Power = Work/Time
 What are two ways you can label power?
 Joules/seconds


work (J) / time (sec)
Watt (W)
 You lift a large bag of flour from the floor to a 1 m
high countertop, doing 100 J of work in 2 sec. How
much power do you use to lift the bag of flour?

Power = Work/Time

Power = 100 J / 2 sec

Power = 50 J/sec

Power = 50 W
James Watt and Horsepower
 What is another unit for power?
 Horsepower (hp)
 Compare this unit with watts.
 1 horsepower = 746 watts
Section 14.1 Assessment
1.
How much work is done when a vertical force acts
on an object moving horizontally?
No work is done, because none of the force acts in the same
direction as the object.
Section 14.1 Assessment
2. A desk exerts an upward force to support a
computer resting on it. Does this force do work?
Explain.
No work is done, because there is no movement.
Section 14.1 Assessment
3. Two cars have the same weight, but one of the cars
has an engine that is twice as powerful as the other.
Which car can make it to the top of a mountain
pass first?
The car w/ the more powerful engine will reach the top first.
Which car does more work to reach the pass?
Both cars do the same amount of work to reach the top.
Section 14.1 Assessment
4. You carry two heavy bags of groceries upstairs to
your kitchen. Will you do more work on the bags if
you carry them one at a time? Explain.
Carrying one bag at a time uses only half the force, but
requires twice the distance.
The work done is the same in both cases.
Section 14.1 Assessment
5. You lift a book to a bookshelf 1 m above the floor.
How much power is used if the upward force is 15
N and you do the work in 2 seconds?
Work = Force x Distance
Work = 15 N x 1 m
Work = 15 J
Power = work/time
Power = 15 J/ 2 sec
Power = 7.5 W
14.2 Work & Machines
CHAPTER 14: WORK, POWER, AND
MACHINES
Machines Do Work
 Define machine:
 A device that changes a force.
 What is the main purpose of machines?
 They make work easier to do.
 In which ways can machines do this?
 Change the size of the force needed.
 Change the direction of the force.
 Change the distance over which a force acts.
Increasing Force
 How can a machine increase a force?
 Increase the distance over which a small force is exerted.
 A small force exerted over a large distance becomes a
large force exerted over a short distance.
 Think of an original example of a machine that
increases force:

Bolt Cutters vs. Scissors
Increasing Distance
 How can a machine increase distance?
 Exert a greater force over a smaller distance.
 A machine that decreases the distance through which
you exert a force, increases the amount of force
required.
 Think of an original example of a machine that
increases distance:


Gears on a bike
Broom, Golf club, Bat
Changing Direction
 How can a machine change the direction of the
applied force?
 Think of an original example of a machine that
changes direction:


Levers
Pry bars
Work Input & Work Output
 How does the work done ON a machine compare to
the work done BY a machine?


Work done ON a machine is bigger than work done BY it
More work goes in than comes out.
 What does friction have to do with this?
 Friction causes work/force to be lost.
 It can be lost as:
Heat
 Light
 Sound

Work Input to a Machine
 Define input force:
 The force you exert on a machine.
 Define input distance:
 The distance over which you exert a force.
 Define work input:
 Work done by you.
 Write an equation that uses the last three terms:
 Work Input = Input Force x Input Distance
Work Output of a Machine
 Define output force:
 The force exerted by a machine
 Define output distance:
 The distance over which a machine exerts a force
 Define work output :
 The work done by a machine
 Look at Figure 7 (pg. 419). What is the input
distance?

The length of the path over which the oar handle moves

(the arc)
 How does the input distance compare to the output
distance?

It is less
Section 14.2 Assessment
1.
A machine produces a larger force than you exert to
operate the machine. How does the input distance
of the machine compare to its output distance?
Because the output force is greater than
the input force, the input distance must
be greater than the output distance
This could be an example of a car jack.
Section 14.2 Assessment
2. You do 200 J of work pulling the oars of a rowboat.
What can you say about the amount of work the
oars do to move the boat? Explain.
The work done by the oars is less than 200 J, because some
input force is always lost to friction.
What kind of friction is involved here?
The friction in this case would be fluid friction.
Section 14.2 Assessment
3. How can you increase the work output of a
machine?
You can increase the work you put into a machine.
You can try to reduce the friction
* Ball Bearings
* Oil/Grease
* Make sure the parts that touch have smooth
surfaces
*…
Section 14.2 Assessment
4. When you swing a baseball bat, how does the
output distance (the end of the bat) compare with
the input distance (the distance your hands move)?
The output distance is greater than the input distance.
Why might it be useful to know this?
You know that if you swing harder, you will increase the
force at the end of the bat, making the ball go further.
Section 14.2 Assessment
5. An ad claims that a new wrench reduces the force
needed to tighten a bolt. If this ad is true, what do
you know about the input distance?
If the input force is reduced, then the input distance must be
increased.
14.3 Mechanical Advantage &
Efficiency
CHAPTER 14: WORK, POWER, AND
MACHINES
Mechanical Advantage
 Define Mechanical Advantage
 The number of times the machine increases an input force
Actual Mechanical Advantage
 Define AMA:
 The ratio of the output force to the input force
 Write the equation:
 AMA = O. Force/ I. Force
Ideal Mechanical Advantage
 Define IMA:
 The MA of a machine in the absence of friction
 Write the equation:
 IMA = I. Distance/O. Distance
 Why is the AMA always less than the IMA?
 Because there is always friction
 What can be done to make the AMA closer to the
IMA?




Use low-friction materials
Ball bearings
Oil/grease
Stream-lining
Calculating Mechanical Advantage
 How do you calculate IMA? Use words & an
equation.

IMA = I. Distance/O. Distance
 What two things do you need to calculate IMA?
 Input Distance & Output Distance
 Why do we calculate IMA instead of AMA?
 It’s easier to measure
 Does this give us the results we want?
 No, it’s what we’d have if there was no friction (in a perfect
world)
Efficiency
 Define Efficiency:
 The percentage of the work input that becomes work output
 Write out the equation:
 Efficiency = Work O. / Work I. X 100%
 Why is efficiency of any machine always less than
100%?

Because there’s always friction.
 What are some ways to increase efficiency?
 Ball bearings
 Grease/Oil
 Smoother surfaces…
 Can you think up a machine that would have 100%
efficiency?
Section 14.3 Assessment
1.
You test a machine and find that it exerts a force of
5 N for every 1 N of force you exert operating the
machine. What is the AMA of the machine?
AMA = O. Force/ I. Force
AMA = 5N / 1 N
AMA = 5
Section 14.3 Assessment
2. How can 2 machines appear identical and yet not
have the same AMA?
They can have different amounts of friction.
Section 14.3 Assessment
3. What information would you use to calculate the
efficiency of a machine?
Work input and Work output
Section 14.3 Assessment
4. When is the IMA of a machine greater than 1?
Whenever output force is greater than input force.
Section 14.3 Assessment
5. Suppose you are an inventor in 1900. You are
constructing a bicycle of your own design. What
could you do to ensure your bicycle efficiently
changes the work input into forward motion?
Reduce friction as much as possible
…
Section 14.3 Assessment
6. You have just designed a machine that uses 1000 J
of work from a motor for every 800 J of useful
work the machine supplies. What is the efficiency
of your machine?
E = (W.O./W.I.) x 100
E = (800J/1000J) x 100
E = 80%
Section 14.3 Assessment
7. If a machine has an efficiency of 40% and you do
1000 J of work on the machine, what will be the
work output of the machine?
E = (W.O./W.I.) x 100
W.O. = (E x W.I.) / 100
W.O. = (40% x 1000 J) / 100
W.O. = 400 J
Section 14.3 Assessment
8. A construction worker moves a crowbar through a
distance of 0.5 m to lift a load 0.05 m off the
ground. What is the IMA of the crowbar?
IMA = I.D. / O.D
IMA = 0.5m / 0.05m
IMA = 10
Section 14.3 Assessment
9. The IMA of a simple machine is 2.5. If the output
distance of the machine is 1 m, what is the input
distance?
IMA = I.D. / O.D.
I.D. = O.D. x IMA
I.D. = 1 m x 2.5
I.D.= 2.5 m
14.4 Simple Machines
CHAPTER 14: WORK, POWER, AND
MACHINES
 What are the 6 types of
simple machines?

Lever

Wedge

Wheel & Axle

Screw

Inclined Plane

Pulley
Levers
 Define lever:

A rigid bar that is free to move around a fixed point
 Define fulcrum:

The fixed point the bar rotates around
 Levers are classified into 3 categories based on the
locations of what?

The locations of the input force, output force, and fulcrum
 Define input arm:

The distance between the input force and fulcrum
 Define output arm:

The distance between the output force and the fulcrum
 How do you calculate the IMA of a lever?
 Divide the input arm by the output arm
First-Class Levers
 Where is the fulcrum located on a first-class lever?
 Between the input and output force
 Draw and label a sketch of a first-class lever
 What is the MA of a first-class lever?
 Greater than, equal to, or less than 1
 Name some examples of simple machines that use
first-class levers:


Teeter-totter
Scissors
Second-Class Levers
 Where is the output force located on a second-class
lever?

Between the input force and fulcrum
 Draw and label a sketch of a second-class lever:
 What is the MA of a second-class lever?
 Greater than 1
 Requires less force, but more distance
 Name some examples of simple machines
that use second-class levers:


Wheelbarrow
Nut cracker
Third-Class Levers
 Where is the input force located on a third-class
lever?

Between the fulcrum and the output force
 Draw and label a sketch of a third-class lever:
 What is the MA of a third-class lever?
 Less than 1
 The output distance is greater then the input distance
 Name some examples of simple machines that use
third-class levers:




Broom
Hockey stick
Baseball bat
Golf club
Wheel & Axle
 Define a wheel and axle:
 A simple machine that consists of 2 disks or cylinders, each w/
a different radius
 Draw and label a sketch of a wheel and axle:
 How do you calculate the IMA of a wheel and axle?

Divide the radius (diameter) of the input force by that of the
output force
 What is the MA of a wheel and axle?

Greater or less than one

Depends which end is the input
 Name some examples of simple machines that use a
wheel and axle:


Steering wheel and column
Screwdriver
Inclined Planes
 Define an inclined plane:
 Slanted surface along which a force moves an object to a
different elevation
 Draw and label a sketch of an inclined plane:
 How do you calculate the IMA of an inclined plane?
 The distance along the plane divided by the change in height
 What is the MA of an inclined plane?
 Greater than 1
 Name some examples of simple machines that use
inclined planes:



Moving ramp
Wheelchair ramp
Switchbacks on a mountain
Wedges
 Define a wedge:
 A V shaped object whose sides are 2 inclined planes sloped
towards each other
 Draw and label a sketch of an wedge:
 What has a greater IMA:

A thin wedge 8 cm long?

A thick wedge 8 cm long?
 Name some examples of simple machines that use
wedges:

Knife blade

Zipper

Axe blade
Screws
 Define a screw:
 An inclined plane wrapped around a cylinder
 Draw and label a sketch of a screw:
 What has a greater IMA:

A screw with close-together threads?

A screw with far-apart threads?
 Name some examples of simple machines that use
screws:

Screws

Nuts

Bolts

Corkscrew
Pulleys
 Define a pulley:
 A simple machine that consists of a rope that fits into a groove
in a wheel
 How do you find the IMA of a pulley or a pulley
system?

It is equal to the number of rope sections supporting the load
being lifted
Fixed Pulleys
 Describe a fixed pulley:
 Wheel attached at a fixed location
 Rotates in place
 Direction of force is changed, but the size of the force is not
 Draw and label a sketch of a fixed pulley:
 What is the MA of a fixed pulley?
 Always 1
 IF = OF (disregarding friction)
 Name some examples of fixed pulleys:
 Flag pole
 Mini-blinds
Moveable Pulleys
 Describe a moveable pulley:
 The pulley is attached to the object being moved
 Draw and label a sketch of a moveable pulley:
 What is the MA of a moveable pulley?
 Greater than 1
 Name some examples of moveable pulleys:
 Sails on a ship
 Skyscraper window washers’ platforms
Pulley Systems
 Describe a pulley system:
 Fixed and moveable pulleys together
 Draw and label a sketch of a pulley system:
 What is the MA of a pulley system?
 Greater than 1
 Even larger than a moveable pulley
 Name some examples of pulley system:
 Train car lifters
Compound Machines
 Define a compound machine:
 A combination of 2 or more simple machines that work
together
 Name some examples of compound machines:
 Car engine
 Washing machine
 Watch
 Scissors
 …